Recently, inexpensive copper powder is being used in place of expensive silver powder as a metal powder for electroconductive paints such as a paint for printed wiring of electronic circuits. Such electroconductive paints (hereinafter referred to simply as conductive paints) are generally produced by mixing dendritic copper powder which is obtained through electrodeposition with synthetic resins and organic solvents. At the time of use, such conductive paints are applied on an insulative substrate by a method such as screen printing to form a conductive circuit. Then the coated film of this circuit is heated to harden, after which a flux is applied on the above-mentioned coating film to solder lead wires.
It is necessary that conductive paints or copper powders used in this manner have improved conductivity, good adhesion of coated film and good solderability or wettability relative to the coated film. As these paints or copper powders, various materials have hitherto been proposed but these have not always been fully satisfactory.
For example, according to the disclosure of Japanese Patent Publication No. 39693/1989, it is important that hydrogen reduction loss does not exceed 0.20% in order to improve the conductivity of the coated film. Accordingly, the oxidation film of dendritic electrolytic copper powder is reduced by a drying process such as that in a high-temperature hydrogen stream. However, in this case, this process is accompanied by such numerous problems in practicality that the danger of explosion or complexity of facilities and process is always involved, and the surface state of the copper powder after treatment is very active, whereby the possibility of re-oxidization tending to increase hydrogen-reduction loss is high. The term herein referred to as "hydrogen-reduction loss" means a value obtained by the measuring method specified in the JPMA P03-1992 method. In the case of a copper powder, the weight loss percentage is measured when the powder is reduced in a hydrogen stream at 875.degree. C. for 30 minutes, and the oxidized film which impedes conductivity is measured as a weight loss percentage.
In addition, Japanese Patent Publication No. 6254/1991 discloses an invention concerning a solderable and conductive paints comprising dendritic electrolytic copper powders, resol type phenolic resins, fatty acids or salts thereof and metallic chelating agents. According to this reference, direct soldering on a coated film is possible whereby further improvement of conductivity is possible. In actual practice, however, this invention has a disadvantage in that direct soldering on the surface of coated film is difficult, resulting in complexity of process-shortening in circuit or coated film forming.
We have carried out extensive studies and experiments directed toward providing a copper powder for solderable and conductive paints having improved solderability and excellent adhesion and conductivity and toward a process for producing the same by solving the above described problems. As a result, we have found that oil absorption and hydrogen-reduction loss have an effect on properties such as solderability, adhesion and conductivity and that if oil absorption and hydrogen-reduction loss are kept below specific limits, these properties can be improved.
That is, we have found that a copper powder of high oil absorption cannot be contained in conductive paints in a high content, and as a result the amount of copper powder on the surface of a coated film is small, giving rise to failure in soldering. We have found that this mainly stems from the shape of the copper particles and from the fact that, in the case of dendritic copper powder obtained by electrodeposition, the branches of the copper powder which have developed into the form of tree branches obstruct the dispersion into the resin with which they are mixed in the process step of producing a paint. We have found that in order to solve this problem, it is necessary that the dendritic copper powder be crushed by means of a grinding apparatus into green caterpillar-like or scalloped crystals. Thus we have attained the present invention.